More than a billion people have travelled on the Tube in the past year - the first time the landmark has been reached in its 144-year history.
Tube bosses face a challenge to meet growing demand
London Underground recorded its billionth passenger of the financial year 2006/07 this week.
In the year 2005/06 the Tube had 971 million users.
King's Cross remains the network's busiest station with nearly 72 million annual passengers, with numbers expected to reach 127 million by 2016.
Mayor Ken Livingstone said the milestone showed how "crucial the Tube is to London's economic success".